Sputtering targets and method for the preparation thereof

ABSTRACT

Sputtering targets comprising sub-stoichiometric titanium dioxide, TiO x , where x is below 2, are provided. The targets are preferably formed by plasma spraying so as to have an electrical resistivity of less than 0.5 ohm.cm.

[0001] The present invention relates to improved sputtering targets and,in particular to sputtering targets of titanium dioxide, and to a methodfor the preparation thereof.

[0002] Sputtered coatings of various oxides (e.g. silica) and nitrides(e.g. silicon nitride) are used to form optical coatings showinginteresting properties on a number of substrates. Known applicationsinclude low emissivity films on window glasses, cold mirrors onreflectors, enhanced mirrors for photocopiers and antireflectivecoatings on picture glass or TV screens. These coatings are usually madeof stacks of several different layers with different refractive indices,preferably of low and high refractive index, to produce optical filters.For antireflective coatings it is preferred to combine two materialsshowing the highest and the lowest possible refractive indices. Suchmaterials are titania and silica. Another advantage of these materialsis their durability.

[0003] Titanium dioxide coatings have a high refractive index and canthus be used to provide coatings of a high refractive index or toprovide the high refractive index coatings in optical stacks. Theexisting process for producing titanium dioxide coatings comprises usingtitanium metal as the sputtering target and using oxygen as a componentof the plasma gas. The titanium is thus converted to titanium dioxideduring the sputtering process. Although satisfactory coatings oftitanium dioxide can be produced, the rate of production is very slowand much slower than coating with silica.

[0004] As a substitute for titanium dioxide it has been suggested to usealternative materials such as niobium oxide. Whilst it is possible tocoat a substrate with niobium oxide using a niobium metal target atslightly higher speeds than the equivalent process using titanium,niobium is very expensive.

[0005] Thus, there is a need for an improved process for coatingtitanium dioxide onto substrate materials. We have now surprisinglydiscovered that titanium dioxide can be sputtered from a targetcomprising sub-stoichiometric titanium dioxide to provide coatings on asubstrate either of sub-stoichiometric titanium dioxide, or titaniumdioxide, depending upon the sputtering conditions.

[0006] Accordingly, the present invention provides a sputtering targetwhich comprises sub-stoichiometric titanium dioxide, TiO_(x), where x isbelow 2. Sub-stoichiometric titanium dioxide, TiO_(x), where x is below2 and generally is in the range of from 1.55 to 1.95 is known in theart. It may be produced by the reduction of stoichiometric TiO₂. It is aform of titanium dioxide which is conductive.

[0007] The sputtering target of the present invention may comprisesub-stoichiometric titanium dioxide, TiO_(x) coated onto a target base.such as a backing tube or plate, for example a target base of anelectrically conductive material, for example stainless steel ortitanium metal. The target may be of any type known in the art, forexample a rotatable target or a flat magnetron target.

[0008] The sputtering target of the present invention may be prepared byplasma spraying titanium dioxide onto a target base. During the plasmaspraying process, the titanium dioxide loses some oxygen atoms from itslattice and is converted into the sub-stoichiometric form. The primaryplasma gas used for the plasma spraying is preferably argon, withhydrogen as the secondary plasma gas. The titanium dioxide which issubjected to plasma spraying preferably has a particle size in the rangeof from 1 to 60 micrometers. It is also important to cool the sputteringtarget during the plasma spraying in order to quench the titaniumdioxide in sub-stoichiometric form and to improve the conductivitythereof. It also important to use a certain amount of hydrogen in theplasma gas in order to produce a high temperature plasma and to assistin the reduction.

[0009] The present invention also provides in another aspect a processfor coating a substrate surface with titanium dioxide, which processcomprises using as a sputtering target a target comprisingsub-stoichiometric titanium dioxide, TiO_(x). The sputtering from thetarget is preferably carried out using as the plasma gas argon, amixture of argon and oxygen, a mixture of nitrogen and argon, or amixture of nitrogen and oxygen. If the plasma gas does not containoxygen, e.g. if pure argon is used, then the coating will comprisesub-stoichiometric titanium dioxide. The coating is not completelytransparent and possesses some conductivity. If, however, the plasma gascontains oxygen then the sub-stoichiometric form of titanium dioxide isconverted into the transparent form which is stoichiometric orsubstantially stoichiometric. The degree of transparency will dependupon the amount of oxygen contained in the plasma gas. A preferred gasmixture to form transparent titanium dioxide as the coating comprises70-90% by volume argon and 30-10% by volume of oxygen.

[0010] The substrate which is coated according to this process maycomprise, for example, optical glass, the screen of a cathode ray tube,such as a TV screen, cold mirrors, low-emissivity glasses, architecturalglasses, antireflective panels, flexible films or oxygen barrier films.In this case the coating process will be carried out under conditionssuch that the sub-stoichiometric titanium dioxide is converted into thestoichiometric form.

[0011] In a further aspect the present invention also provides a processfor the preparation of sub-stoichiometric titanium dioxide, TiO_(x),where x is below 2 which process comprises subjecting titanium dioxideto a plasma flame. In carrying out this process the titanium dioxide ispreferably sprayed through a plasma flame, for example a plasma flameusing a mixture of argon and hydrogen as the plasma gas.

[0012] The main advantage of the present invention is that from thesub-stoichiometric titanium dioxide targets used in the presentinvention the rate of sputtering is increased by a factor of about tenas compared to sputtering from a titanium metal target, thus making theprocess industrially attractive.

[0013] The present invention will be further described with reference tothe following Examples.

EXAMPLE 1 Comparative

[0014] A rotatable target comprising a tube of titanium metal ofdiameter 133 mm and length 800 mm was used to sputter titanium metalonto a glass plate placed at a distance of 18 cm from the target. Thesputtering was carried out at a power level of 35 kW (80A, 446V) under apressure of 5×10⁻³ mBar of argon as the plasma gas.

[0015] After 3½ minutes a layer of titanium metal 18000 Angstroms inthickness as measured by a profilometer had been deposited upon theglass plate.

EXAMPLE 2 Comparative

[0016] The procedure of Example 1 was repeated but substituting amixture of 80% O₂ and 20% Ar as the primary plasma gas to replace theargon primary plasma gas of Example 1. The sputtering was carried out ata power level of 45 kW (97A, 460V) under a pressure of 4.5×10⁻³ mBar.Using a titanium metal target as described in Example 1 a titaniumdioxide layer of thickness 1500 Angstroms was deposited on a glass plateplace above the target in 3½ minutes.

EXAMPLE 3

[0017] A rotatable target comprising a tube of stainless steel ofdiameter 133 mm and length 800 mm was coated with sub-stoichiometrictitanium dioxide, TiO_(x), where x is below 2 as hereinbefore describedby plasma spraying titanium dioxide onto the target using argon as theprimary plasma gas and hydrogen as the secondary plasma gas. 72 liters(60% argon, 40% hydrogen) were used. The power level was 45 kW (455A,96V).

[0018] This target was then used as a sputtering target in the manner asdescribed in Example 1. Using argon as the primary plasma gas thesputtering was carried out at a power level of 45 kW (97A, 460V) under apressure of 5.4×10⁻³ mBar Ar. A dark blue semitransparent layer ofsub-stoichiometric titanium dioxide, TiO_(x), of thickness 14000Angstroms was deposited on a glass plate placed above the target in 3½minutes. The sputtering proceeded smoothly without significant arcing.

EXAMPLE 4

[0019] A rotatable target prepared as described in Example 3 was used asa sputtering target in the manner as described in Example 3 using amixture of 75% Ar and 25% O₂ as the plasma gas. The sputtering wascarried out at a power of 45 kW (95A, 473V) under a pressure of 5×10⁻³mBar. A clear transparent coating of stoichiometric titanium dioxide ofthickness 12500 Angstroms was deposited on a glass plate placed abovethe target in 3½ minutes. The sputtering proceeded smoothly withoutsignificant arcing.

EXAMPLE 5 Comparative

[0020] A rotatable target was prepared as described in Example 3 byusing pure argon (40 liters) at a power level of 34 kW (820A, 42V). Theelectrical conductivity of the target was ten times inferior to that ofExample 3. Sputtering from the target was difficult due to arcing. Theprocess was not stable enough to produce samples.

1. A sputtering target which comprises sub-stoichiometric titaniumdioxide, TiO_(x), where x is below
 2. 2. A sputtering target as claimedin claim 1 wherein the TiO_(x) is coated onto an electrically conductivebase.
 3. A process for the preparation of a sputtering target as claimedin claim 1 or claim 2, which process comprises plasma spraying titaniumdioxide, TiO₂, onto a target base.
 4. A process as claimed in claim 3wherein the target base is cooled during the plasma spraying.
 5. Aprocess as claimed in claim 3 or claim 4 wherein the plasma spraying iscarried out using argon as the plasma gas and hydrogen as the secondaryplasma gas.
 6. A process as claimed in any one of claims 3 to 5 whereinthe target bas e is titanium.
 7. A process as claimed in any one ofclaims 3 to 6 wherein the titanium dioxide which is plasma sprayed hasparticle size in the range of from 1 to 60 micrometers.
 8. A process forcoating a substrate surface with titanium dioxide, which processcomprises the use as a sputtering target of a target as claimed in claim1 or claim
 2. 9. A process as claimed in claim 8 wherein the sputteringfrom the target is carried out using as the plasma gas argon, a mixtureof argon and oxygen, a mixture or argon and nitrogen, or a mixture ofnitrogen and oxygen.
 10. A process as claimed in claim 9 wherein theplasma gas comprises 70 to 90% by volume argon and 30 to 10% by volumeoxygen.
 11. A process as claimed in any one of claims 8 to 10 whereinthe substrate which is coated is optical glass, the screen of a cathoderay tube, a flexible film or an oxygen barrier film.
 12. A substratewhich has been coated by a process as claimed in any one of claims 8 to11.
 13. A process for the preparation of sub-stoichiometric titaniumdioxide, TiO_(x), where x is below 2 which process comprises subjectingtitanium dioxide to a plasma flame.
 14. A process as claimed in claim 13wherein titanium dioxide is sprayed through a plasma flame.
 15. Aprocess as claimed in claim 13 wherein the titanium dioxide is sprayedusing a combustion flame.